Nanotechnology is being applied to a diverse array of products, ranging from cosmetics, printer toners, clothing, electronics, and even drug delivery vehicles. Carbon nano-materials, such as fullerenes and nanotubes, have been the most extensively used nanoparticles due to their unique and superior physical and chemical properties, including large surface areas, high electrical conductivity, and excellent mechanical strength. Fullerenes, also called C60 or buckyballs, and other fullerene derivatives, are the most well studied and most commonly used carbon nanomaterials. Recently, fullerenes were investigated as potential microbicides; other potential in vivo applications were explored as well. However, the toxicological definition for fullerene is still quite controversial. Early studies have indicated that a repeating subchronic topical dose of fullerenes on mouse skin for up to 24 weeks is non-carcinogenic (Nelson, M. A.; Domann, F. E.; Bowden, G. T.; Hooser, S. B.; Fernando, Q.; Carter, D. E. Toxicol Ind Health 1993, 9, (4), 623-30). The Ames assay also indicates that the fullerene is not mutagenic and of no toxicological significance (Mori, T.; Takada, H.; Ito, S.; Matsubayashi, K.; Miwa, N.; Sawaguchi, T. Toxicology 2006, 225, (1), 48-54). Yet, recently, fullerenes have been suggested to be carcinotoxic and genotoxic, although only upon photosensitization. In addition, C60 derivatives have demonstrated superoxide dismutase mimetic properties; they can also generate free radicals, can be photosensitized, and mutagenic. In contrast, others indicate that fullerenes have avid reactivity with free oxidative radicals, acting as radical scavengers and antioxidants instead.
In general, water-soluble fullerenes are cytotoxic, which can be attenuated by surface derivatization (Sayes, C. M.; Fortner, J. D.; Guo, W.; Lyon, D.; Boyd, A. M.; Ausman, K. D.; Tao, Y. J.; Sitharaman, B.; Wilson, L. J.; Hughes, J. B.; West, J. L.; Colvin, V. L. Nano Letters 2004, 4, (10), 1881-1887). However, a recent report demonstrated the opposite results (Chiron, J.; Lamande, J.; Moussa, F.; Trivin, F.; Ceolin, R. Ann Pharm Fr. 2000, 58, (3), 170-175). Isakovic et al, in Biomaterials 2006, 27, (29), 5049-58, suggested that the trace amount of THF in the fullerene toxicity studies was responsible for the cytotoxicity. It has also been shown that cationic fullerenes arc moderately toxic (Bosi, S.; Da Ros, T.; Castellano, S.; Banfi, E.; Prato, M. Bioorg Med Chem Lett 2000, 10, (10), 1043-5) and it has been suggested that these fullerenes affect energy metabolism. Mashino, T.; Nishikawa, D.; Takahashi, K.; Usui, N.; Yamori, T.; Seki, M.; Endo, T.; Mochizuki, M. Bioorg Med Chem Lett 2003, 13, (24), 4395-7. In contrast, anionic fullerenes have been shown to be relatively less toxic by some reports, yet other reports indicate that anionic fullerenes can inhibit bacterial growth, (Tsao, N.; Luh, T. Y.; Chou, C. K.; Wu, J. J.; Lin, Y. S.; Lei, H. Y. Antimicrob Agents Chemother 2001, 45, (6), 1788-93) more specifically that anionic fullerene derivatives (carboxyfullerene) affect Gram positive bacteria (such as Streptococcus pyogenes), but have no affect on Gram negative bacteria (such as Escherichia coli) at concentrations up to 500 mg/L. (Tsao, N.; Luh, T. Y.; Chou, C. K.; Chang, T. Y.; Wu, J. J.; Liu, C. C.; Lei, H. Y. J Antimicrob Chemother 2002, 49, (4), 641-9). The strong cytotoxicity of cationic fullerene compounds (e.g., ammonium or other amino acid-derivatized fullerene) has been shown in many microorganisms. It has also been shown that the presence of light-induced reactive oxygen species (ROS) enhance fullerene anti-microbial activity. The current hypothesized nanotoxicity mechanisms include suppression of energy metabolism (e.g. TCA cycle), oxidative damage to crucial proteins and enzymes, and increased membrane permeability, causing its rupture. (Jensen, A.; Wilson, S.; Schuster, D. Bioorg Med Chem Lett 1996, 9, (20), 2959-62; Sayes, C.; Gobin, A.; Ausman, K.; Mendez, J.; West, J.; Colvin, V. Biomaterials 2005, 26, (36), 7587-95). Since mechanisms of cytotoxicity obtained from animal/human cell models may not be compatible with microbial models, the exact molecular mechanisms for the inhibition of bacterial growth are still not fully understood. Furthermore, the majority of nanoparticle antibacterial experiments were not performed at the molecular and metabolic levels, which are central for bacterial survival and proliferation.